Gas transport system for powders

ABSTRACT

A gas transport system for powder has a delivery pipe having an end portion of porous material having a pore size smaller than the particles of the powder. The porous end portion is surrounded by a jacket in which a vacuum is drawn to draw off transport gas and reduce filter capacity which would otherwise be required to remove it. Pulses of pressure are periodically introduced into the jacket to clear particles from the inside of the porous end portion. The vacuum is drawn in the jacket through a vacuum line containing a constriction through which a nozzle discharges high pressure gas at intervals to provide the pulses of pressure in the jacket. This system is particularly effective when used in the production of uranium oxide.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the air slide delivery of powders of all typesand to the transport of particulate matter by a gas.

2. Description of the Prior Art

Air slide systems with filters to remove transport air from the vesselto which the particulate matter is delivered are old. These systemsoften require costly "bag house" filters to handle the large volumes oftransport air involved.

SUMMARY OF THE INVENTION

An air slide or gas transport system for particulate matter dischargesinto a vessel from which the transport gas is removed through filters.The discharge tube has a vertical end portion of porous materialsurrounded by a jacket in which a lower pressure is drawn. The lowerpressure in the jacket draws off transport gas through the porous endportion of the delivery tube reducing the load on the filters and thusrequired filter capacity. At least the inner wall of the porous endportion has a pore size smaller than the particulate matter beingtransported so that only transport gas is withdrawn through the jacket.

The rush of gas and particulate matter through the porous dischargeportion prevents the caking of powder therein. At periodic intervals ahigh pressure pulse is introduced into the jacket to blow back throughthe porous end portion and clear its pores. The pulses of pressure maybe provided by directing a jet of high pressure gas back through aconstriction in a vacuum line leading from the jacket.

The use of the porous discharge portion is far more efficient in theremoval of transport gas than conventional filters in the vessel towhich the powder is delivered as the velocity of the particles passingthrough the tubular porous end portion prevents them from impingingdirectly on the porous wall. Thus for its area and a given differentialor driving pressure, the porous end portion removes transport gas moreefficiently than any equivalent conventional filter.

In the production of uranium oxide by the burning of uraniumhexafluoride the size of the filters are limited by the volumedetermined by the critical mass of uranium that can exist within anygiven reactor. Conventional filtration practice limits air flow to 4 to6 cubic feet per minute per square foot of filter material for goodfilter practice if a blow back system to periodically clear the filtersis to be effective. Thus the production rate of the process is limitedby filter capacity. The use of the system of this invention permits asubstantial increase in the production of uranium oxide withoutincreasing the load on the filters and with no change in the criticalvolume of the reactors themselves.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is an elevational view, partly in section and with centralportions of elements broken away, of a gas transport system for powder;

FIG. 2 is a vertical section through the end of a vacuum line connectedwith a "T" to a fragment of a jacket, the "T" containing a jet for highpressure gas and a constriction for intermittently pressurizing thejacket; and

FIG. 3 is a section through a fragment of a porous wall of an endportion of a delivery tube.

DESCRIPTION OF THE PREFERRED EMBODIMENT

As shown in FIG. 1, a delivery turbine 10 draws powder and air or anyother suitable transport gas through pipe 11 and blows it throughdelivery pipe 12 into tank 13. Tank 13 contains filter 14 connected to avacuum pump 15 by tubes 16, 17 and 18. The forementioned elements arewell known in the gas transport of particulate matter.

An end portion 20 of pipe 12 is of porous material surrounded by ajacket 21. End portion 20 is best disposed vertically. Vacuum turbine 22draws a vacuum through vacuum line 23 in jacket 21. As powder 24 and itstransporting gas 25 reach tank 13, the transporting gas 25 is drawn offthrough filters 14 leaving the particulate matter 24 in container 13. Inconventional air slide or other gas transport systems, filter capacityto remove all the air or other transport gas is required.

In the practice of this invention, the vacuum jacket 21 draws offtransport gas 25 through the porous end portion 20 reducing the requiredfilter capacity in tank 13 as much as fifty percent. The pore size ofportion 20 should be small enough to prohibit passage of any of theparticulate matter 24 being transported. Although caking of powder 24 inportion 20 is inhibited as cake is knocked away by high speed passingparticles as it is formed, eventually the pores of the portion 20 maytend to clog.

A source of gas 30 under pressure, such as a tank of CO₂, delivers gasthrough a pressure reducing valve 31, line 32, and solenoid valve 33.Valve 33 is periodically opened for short intervals by a timer 34 todirect gas through tube 41 to nozzle 35 to blast through constriction 36in vacuum line 23. This arrangement is best accomplished using a "T"fitting 37 with the nozzle 35 and constriction 36 in or beyond the arms38 and 39 of the "T" 37 which has line 23 connected to its leg 40. Eachshort duration blast of gas through constriction 36 draws additional gasback through line 23 to pressurize jacket 21 and clear the pores ofporous end portion 20.

As shown in FIG. 3, portion 20 is best formed with a coarse, large poreouter layer 50 of a pore size larger than the particulate matter orpowder 24 being transported and a thin inner layer 51 with asufficiently small pore size to prevent passage of the powder 24 beingtransported. This construction permits clearing pressure bursts injacket 21 to reach the inner surface layer 51 more readily to moreeffectively clear it with a shock effect. Outer layer 50 mainly servesto support layer 51. Layer 50 may be of rolled screen wire coated with athin layer 51 of sintered porous metal.

The application of this invention to the production of uranium oxide cangreatly increase the production of existing facilities. It can alsogreatly reduce the high cost of operation of "bag house" filters in theair delivery of powder products.

In one test of this invention, powdered coal was air transported and therequired conventional delivery air filter capacity was reduced up to 30percent. A vacuum of five inches of water was maintained in jacket 21surrounding a porous end portion 20 of 1.0 inch I.D., 1.25 inches O.D.,and 25 inches long having a pore size of 10 microns. The pores ofportion 20 were cleared every 20 seconds by three 0.5 second air blastsat 100 p.s.i. during an interval of 3 seconds.

I claim:
 1. In a gas transport system for particulate matter having atank, a delivery pipe, means blowing gas and particulate matter beingtransported thereby through said pipe into said tank, and filter meanswithin said tank through which at least some transport gas is removedfrom said tank; the improvement comprising, in combination, an endportion of porous metal of said delivery pipe, said end portion having athin inner layer with a pore size smaller than the particulate matterbeing transported and an outer layer having a pore size larger than theparticulate matter being transported, said outer layer surrounding andsupporting said thin inner layer, said end portion extending verticallydownward into said tank, a jacket disposed about said porous endportion, a vacuum turbine, a "T" fitting having a first arm connected tosaid jacket, a second arm, and a leg, a constriction in said first arm,a vacuum line connected between said vacuum turbine and said leg, saidvacuum turbine drawing a vacuum in said jacket through said leg, saidfirst arm, and said constriction and thereby drawing at least sometransporting gas outward through said inner and outer layers of said endportion, a nozzle in said second arm of said "T" fitting directed towardsaid constriction in said first arm, a source of gas under pressure, andvalve means incorporating a timer and being connected between saidsource of gas under pressure and said nozzle periodically releasingpulses of gas from said source of gas under pressure to blast from saidnozzle through said constriction against flow therethrough momentarilypressurizing said jacket causing momentary inward flow through saidporous end portion clearing the pores of said inner layer of said porousend portion.